What the angel's share is
Oak is not airtight. Water and alcohol slowly pass through the staves and evaporate, and the warehouse air smells of whisky. How much leaves depends on temperature, humidity, cask size and how the casks are stored. Rates of around 2% of the volume per year are often quoted for cool Scottish warehouses, while hot climates such as Kentucky summers or India and Taiwan lose clearly more, in places double-digit percentages in the early years. Young spirit tends to lose more than old whisky, and a new cask also soaks up spirit into the wood in the first year.
The compound formula
Because each year's loss is a share of what is still in the cask, the right model is compound decay, the same maths as a loan balance or radioactive decay:
LPA after n years = LPA at fill × (1 − rate)^n
In Excel, with the LPA at fill in B4, the rate in B5 and the year in A13: =$B$4*(1-$B$5)^A13, filled down. The total share lost after n years is 1 − (1 − rate)^n: 21.5% after 12 years at 2%, not the 24% that a straight-line 12 × 2% suggests. The straight-line shortcut overstates the loss more the longer the whisky ages.
Worked example: one cask in two warehouses
The cask and the two loss rates are inputs. The table follows the pure alcohol and the bottles it would fill year by year, and the results compare the two warehouses.
| A | B | C | D | E | |
|---|---|---|---|---|---|
| 1 | Cask and assumptions | Value | |||
| 2 | Cask fill (litres) | 250 | |||
| 3 | Fill strength (ABV) | 63.5% | |||
| 4 | Litres of pure alcohol (LPA) at fill | 158.75 | |||
| 5 | Angel's share per year, cool warehouse | 2.0% | |||
| 6 | Angel's share per year, warm warehouse | 4.0% | |||
| 7 | Bottle size (litres) | 0.70 | |||
| 8 | Bottling strength (ABV) | 46% | |||
| 9 | LPA per bottle | 0.322 | |||
| 10 | Loss threshold | 20% | |||
| 11 | |||||
| 12 | Year | LPA left (2%) | Bottles (2%) | LPA left (4%) | Bottles (4%) |
| 13 | 0 | 158.75 | 493 | 158.75 | 493 |
| 14 | 1 | 155.58 | 483 | 152.40 | 473 |
| 15 | 2 | 152.46 | 473 | 146.30 | 454 |
| 16 | 3 | 149.41 | 464 | 140.45 | 436 |
| 17 | 4 | 146.43 | 455 | 134.83 | 419 |
| 18 | 5 | 143.50 | 446 | 129.44 | 402 |
| 19 | 6 | 140.63 | 437 | 124.26 | 386 |
| 20 | 8 | 135.06 | 419 | 114.52 | 356 |
| 21 | 10 | 129.71 | 403 | 105.54 | 328 |
| 22 | 12 | 124.57 | 387 | 97.27 | 302 |
| 23 | 15 | 117.25 | 364 | 86.06 | 267 |
| 24 | 18 | 110.35 | 343 | 76.14 | 236 |
| 25 | |||||
| 26 | Result | Cool (2%) | Warm (4%) | ||
| 27 | Share of alcohol lost after 12 years | 21.5% | 38.7% | ||
| 28 | Straight-line estimate after 12 years (too high) | 24.0% | 48.0% | ||
| 29 | Years until the loss threshold is reached | 11.0 | 5.5 | ||
| 30 | Bottles lost by year 12 | 106 | 191 |
- B4
=B2*B3LPA at fill - B22
=$B$4*(1-$B$5)^A22LPA left after 12 years at 2% - C22
=B22/$B$9Bottles at 46%, 70 cl - B27
=1-B22/$B$4Share lost - B29
=LN(1-$B$10)/LN(1-B5)Years until 20% is gone
Illustrative cask and loss rates, real warehouses vary from year to year. No brand affiliation, not financial or tax advice. Please drink responsibly.
Why model litres of pure alcohol
Distilleries track casks in litres of pure alcohol (LPA = litres × ABV) because alcohol is what they sell, bottle and pay duty on. Volume and strength do not fall in step: in damp warehouses more alcohol than water escapes and the strength drifts down, in dry, warm ones more water escapes and the strength can even rise. Measuring the LPA avoids the question. To turn LPA into bottles, divide by the LPA per bottle: a 70 cl bottle at 46% holds 0.7 × 0.46 = 0.322 LPA, so 158.75 LPA fill about 493 bottles.
How long until 20% is gone?
Solve (1 − rate)n = 1 − loss for n with logarithms:
=LN(1-loss)/LN(1-rate)
At 2% a year the first 20% has evaporated after 11.0 years, at 4% after 5.5 years. A doubled rate roughly halves the time, which is why warm-climate whiskies are often bottled younger. Every extra year in the cask costs alcohol, so an older bottling has to earn a higher price just to stand still.
What it costs the distillery
In the example a cool warehouse loses about 106 bottles of a possible 493 over 12 years, and a warm one about 191. On top of that, the stock ties up cash and warehouse space for the whole period. When you value casks, the cost of the spirit that is left per LPA rises every year, because the same production cost is spread over less alcohol. For the strength and pure alcohol arithmetic, see the ABV formula from OG and FG.
Common mistakes
- Using a straight-line loss. Fill × (1 − rate × years) treats every year's loss as a share of the original fill and overstates the loss.
- Tracking volume instead of pure alcohol. Strength changes while the whisky ages, so litres alone do not tell you how many bottles you can fill.
- Dividing LPA by the bottle size. A 70 cl bottle at 46% holds 0.322 LPA, not 0.7. Divide by bottle size × bottling strength.
- Applying one rate to every warehouse and year. Climate, cask size and the position in the warehouse change the rate. Use your own measured losses where you have them.
- Forgetting the first-year soak. A fresh cask absorbs spirit into the wood, so first-year losses are often higher than later ones.
Frequently asked questions
How do you calculate the angel's share?
Use compound decay: amount left = amount at fill x (1 - annual rate) ^ years. The share lost is 1 - (1 - rate) ^ years. At 2% a year, 21.5% of the alcohol is gone after 12 years.
What percentage is the angel's share per year?
Around 2% of the volume per year is a commonly quoted figure for cool Scottish warehouses. Warmer climates lose more, often around 4% or higher, and very hot regions can lose double-digit percentages in the early years.
How much whisky is lost to the angel's share over 12 years?
At 2% a year about 21.5% of the original alcohol, at 4% a year about 38.7%. For a 250-litre cask at 63.5% that is roughly 106 or 191 bottles of 70 cl at 46%.
Does the angel's share lower the ABV?
It depends on the warehouse. In humid conditions more alcohol evaporates than water and the strength falls. In dry, warm conditions more water evaporates and the strength can rise. That is why distilleries track litres of pure alcohol.